Electrowetting displays are becoming attractive to an ever increasing extent, mainly because of a combination of high brightness, a high contrast ratio, a large viewing angle and a fast switching speed. These properties make electrowetting displays suitable for video applications. Furthermore, the power consumption of electrowetting displays is relatively low, because electrowetting displays use the principle of a reflective display device, i.e. electrowetting displays do not require front or backlight.
An electrowetting display typically comprises a closed electrowetting cell, a polar and non-polar liquid, such as water and a colored oil, having different optical properties and being contained in the cell, a number of electrodes for controlling the liquids contained in the cell, a front layer and a rear reflective layer. The liquids, which are immiscible, may be displaced by means of applying voltages to the electrodes. In an equilibrium-state (in which no voltages are applied to the electrodes) the polar and non-polar liquids are naturally layered in the closed cell, whereby a thin film is created. In this state, a colored off-state, the film covers the reflective area and the cell or pixel appears dark or black. By applying a voltage across the electrodes, the layered off-state is no longer energetically favorable and the cell or pixel may lower its energy by contracting the polar liquid. As a result the non-polar liquid is displaced and the underlying reflective or white surface is exposed. Consequently, in this state, a white on-state, the cell or pixel appears white or bright. The interaction between electrostatic and capillary forces determines how far the non-polar liquid is displaced to the side. In this manner, the optical properties of the layered composition may be adjusted such that intermediate color states, i.e. states lying between the colored off-state and the white on-state, are achieved.
Patent application publication WO 2005/036517 A1 discloses methods of driving devices for optical switches, in particular displays based on the principle of electrowetting. In WO 2005/036517 A1, there are disclosed optical switches comprising a first fluid and a second fluid immiscible with each other within a space between a first transparent plate and a second support plate, the second fluid being electro-conductive or polar. A method of driving the display by means of a reset pulse is disclosed, which brings the pixels of the display device into one of their extreme states (i.e. on or off). When driving an electrowetting display device of this type, each row must accordingly be selected twice each frame. A first selection signal is resetting the pixels and a second selection signal is writing data to the pixels. Even though the resetting of the pixels improves grey scale rendering, the method has the disadvantage of making the frame time or addressing time unnecessarily long.